Nuclear magnetic resonance (NMR) spectrometers typically include a superconducting magnet for generating a static magnetic field B0, and one or more special-purpose radio-frequency (RF) coils for generating a time-varying magnetic field B1 perpendicular to the field B0, and for detecting the response of a sample to the applied magnetic fields. Each RF coil can resonate at the Larmor frequency of a nucleus of interest present in the sample. The resonant frequency of interest is determined by the nucleus of interest and the strength of the applied static magnetic field B0. The RF coils are typically provided as part of an NMR probe, and are used to analyze samples situated in test tubes or flow cells. Conventional NMR magnets and RF coils are characterized by cylindrical symmetry. The direction of the static magnetic field B0 is commonly denoted as the z-axis, while the plane perpendicular to the z-axis is commonly termed the x-y or θ plane. In the following discussion, the term “longitudinal” will be normally used to refer to the z-direction, while the term “transverse” will be used to refer to the x and y directions.
Conventional RF coils used for NMR include helical coils, saddle coils, and birdcage resonators. Conventional RF resonators employed in liquid NMR spectroscopy are designed with care to produce a large cylindrically-symmetric volume of RF magnetic field homogeneity. For information on various NMR systems and methods see for example U.S. Pat. Nos. 4,398,149, 4,388,601, 4,517,516, 4,641,098, 4,692,705, 4,840,700, 5,192,911, 5,818,232, 6,201,392, 6,236,206, and 6,285,189.
An NMR probe can include multiple NMR coils, each tuned for performing NMR measurements on a different nucleus of interest. For example, an NMR probe can include one coil for performing NMR measurements on protons, and another coil for performing NMR measurements on other nuclei of interest, such as 13C or 15N. In such an NMR probe, the design of one coil can affect the performance of the other coil(s). In order to reduce the coupling between two coils, the coils can be disposed in a quadrature configuration, so that the magnetic fields generated by the coils are mutually orthogonal. This configuration minimizes the mutual inductance between the coils.
The measurement sensitivity that can be achieved with an NMR coil increases with the coil quality factor Q and its filling factor n. The quality factor Q can be maximized by reducing coil and sample losses. The filling factor n can be increased by reducing the coil size relative to the sample. At the same time, reducing the coil size relative to the sample can increase magnetic field inhomegeneities. Inhomogeneities in the RF magnetic field adversely affect the measurement sensitivity. Moreover, the coil design and dimensions are constrained by the requirement that the coil resonate in a desired frequency range.
In U.S. Pat. No. 5,552,709, Anderson proposed a method of reducing electrical losses in the sample through the use of special sample cells that compartmentalize the sample. The volume of a cylindrical sample cell is broken up into a plurality of tubular compartments separated by electrically-insulating material. The insulating material reduces the electrical current paths within the sample. The transverse cross-sections of the partitions can be shaped as circles, triangles, rectangles, squares, or sections of cylindrical shells. The described method can be difficult to implement in practice.